I'd like to convert my 242 to use the newer style electronic speedometer
that uses a tone wheel in the rear differential assembly to read the
speed. Does anyone know if I can simply install the tone wheel on my
existing differential and replace the cover with one that has the
sensor? Failing that, can I swap in a differential that has the tone
wheel without a lot of screwing around, or is it easier to just swap the
whole axle assembly?

I realized in thinking about this recently that the rear axle is quite
possibly the only part of a 240 that I've never disassembled.